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Pixel defects

The terms dead, hot and warm pixels are used to describe pixel defects
usually caused in the manufacturing process of a sensor.

A dead pixel
can have a very low output, independent of the amount of incoming
light. A hot pixel can reach saturation even when no photon reaches the
pixel. Warm pixels respond to the incoming light, but output different
values that have no connection to the incoming light.

Most sensors have some defective pixels by their nature, which are
listed in a blemish specification given by the sensor manufacturer.
Some sensor manufacturers supply a 'map' of these defective pixels to
the camera manufacturer so that substitute values from neighbouring
pixels can be used.

Defective pixels can also form after the sensor has left the factory. It has
been shown that cosmic rays can cause pixels to become defective,
especially when the sensor is transported by air.